This project will study the adaptive characteristics of the vestibulo- ocular reflex (VOR) in normal and vestibulopathic human subjects. The vestibular system is crucial for maintaining orientation in space. One of its most important functions is the VOR. This reflex generates eye movements to compensate for head movements, allowing for the maintenance of binocular fixation on visual targets, and hence a stable retinal image. The VOR is the most direct behavioral output from which mechanistic inferences about vestibular function can be derived. It can be functionally divided into two subtypes, the angular VOR (AVOR), driven by the semicircular canals during head rotation, and the linear VOR (LVOR), driven by the otolith organs during linear accelerations. An important attribute of the VOR is its ability to modify performance in response to momentary changes in conditions (e.g. fixation distance, imagined target motion), and to prolonged visual-vestibular mismatch during head movements arising from disease, development and aging. The focus of this study will be to determine whether the various AVOR and LVOR components are under independent adaptive control, or share neural elements. To this end, studies of VOR plasticity will be conducted on normal subjects using optical conditioning (base-out prisms, periscopic and thick-lens optics). We will quantify potential selective adaptation of specific subsets of the VOR (AVOR or LVOR) and specific influences of fixation distance and gaze eccentricity. VOR plasticity will also be studied in patients following unilateral vestibular ablation. The time course of VOR recovery (vestibular compensation) will be characterized. The relationship between optically and surgically induced ablation phenomena will be elucidated, yielding important contributions to our understanding of clinical and basic vestibular physiology.